Electronic devices increasingly include display screens as part of user interfaces. Display screens may be employed in a wide array of devices, including desktop computer systems, notebook computers, and handheld computing devices, as well as various consumer products, such as cellular phones and portable media players. Electronic devices also may include backlights that illuminate the display screens. Ambient light may reflect off the surface of display screens and may reduce the display contrast, thereby making it difficult to view the display screens in high ambient light conditions. Accordingly, as ambient light conditions change, the brightness of a backlight also may be changed to provide sufficient contrast between the ambient light and the backlight.